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We studied temporal fluctuations in fruit production by plants and in populations of understory fruit—eating birds at three elevations (50, 500, and 1000 m) in Costa Rican wet forests over a 12—16 mo period to investigate effects of resource variation on bird movements and community structure. We used mist nets to monitor changes in frugivore abundance, migration patterns, and breeding and molting cycles. We sampled understory fruits of each forest concurrent with studies of frugivores. Both frugivores and fruit exhibited considerable seasonal variation in abundance. Peak frugivore capture rates occurred during peak periods of ripe fruit abundance. Altitudinal migrants left lower montane (1000 m) forest during periods of fruit scarcity and wee present in lowland (50 m) and foothill (500 m) forest when ripe fruit was abundant. Migrants, both altitudinal and temperate, accumulated fat before migration, and perhaps (for altitudinal migrants) in anticipation of breeding. Some residents also put on fat before breeding. Breeding was seasonal at all forests and occurred when ripe fruit abundance was low. Results of this study indicate that birds may track changes in resource abundance. Thus, variation in resource abundance influences dynamics of bird communities, both in terms of species composition and abundance. Further, results illustrate the importance of viewing communities from different scales; dynamics at a local scale (e.g., one elevation) can be influenced by changes in conditions (e.g., fruit abundance) elsewhere. That some species regularly moved along elevational gradients implies that preservation of many species and of the biotic integrity of entire systems may require conservation of large, connected blocks of suitable habitat.